Electroluminescent device

ABSTRACT

An electroluminescent device ( 13 ), such as a light emitting diode, which has a light-reflecting surface ( 10 ) causing undesirable reflection of ambient light incident on the device is provided with a combination of a reflective circular polarizer ( 17 ) and an absorbing circular polarizer ( 23 ) to suppress the undesirable reflection of ambient light thus improving the contrast of the device when used under high intensity ambient lighting conditions while maintaining a satisfactory brightness. The reflection band of the reflective circular polarizer regions ( 17 ′) of the reflective circular polarizer ( 17 ) are preferably tuned to the corresponding emission band of the luminescent regions ( 9 ′) of the electroluminescent device to further increase the contrast of the device while substantially maintaining the same brightness.

The invention relates to an electroluminescent device.

Electroluminescent (EL) devices are devices which emit light whenconnected to a voltage source supplying a suitable voltage. Ofparticular interest are organic electroluminescent devices, or more inparticular polymer electroluminescent devices in which thelight-emissive layer is made of organic or polymeric materialrespectively.

Electroluminescent devices are suitable for display, lighting andsignage applications. Organic (polymer) electroluminescent devices arein particular suitable if the device is to be thin and/or to emit lightacross a large surface area.

WO 97/38452 discloses an electroluminescent device in the form of adisplay. The display has a light-emissive layer and a light-reflectingback electrode. The back electrode reflects ambient light towards theviewer and because such reflected ambient light does not contain anypicture information, the contrast of the displayed picture is adverselyaffected. According to WO 97/38452, in order to avoid such loss ofcontrast a circular polarizer is arranged in front of the display. Thecircular polarizer absorbs all ambient light incident on the display,while allowing light emitted by the light-emissive layer to betransmitted thus improving the contrast of the display in particularwhen ambient light intensity is high.

A disadvantage of the said known display is that the improvement incontrast results in a significantly lower brightness and/or efficiency(if the same brightness is to achieved the drive current must beincreased and thus more power is consumed) of the display because thecircular polarizer absorbs about half of the unpolarized light emittedby the light-emissive layer. Such loss of brightness and/or efficiencyis unacceptable, in particular when the display is battery-operated.

It is an object of the invention, inter alia, to eliminate or at leastmitigate the loss of brightness and/or efficiency caused by the circularpolarizer. More particular, it is an object to provide anelectroluminescent device which combines a good contrast, even underhigh ambient lighting conditions, with a satisfactory brightness and/orefficiency.

This object is achieved by means of an electroluminescent devicecomprising: an electroluminescent layer having luminescent regions i=1to N, with N>=1, each adapted to emit light within an emissionwavelength range Δλ_(EL)(i);

a light-reflecting surface comprising one or more light-reflectingsurface regions arranged, with a light-reflecting side, oppositerespective ones of said luminescent regions, the said surface regionsbeing capable of inverting the handedness of a circularly polarizedcomponent of light incident on the surface regions;

arranged on a side of the electroluminescent layer facing away from thelight-reflecting surface, an absorbing circular polarizer adapted toabsorb one circularly polarized light component and transmit the lightcomponent orthogonal thereto; and

arranged between the absorbing circular polarizer and theelectroluminescent layer, a reflective circular polarizer comprisingreflective circular polarizer regions i=1, . . . , N, with N>=1, thei-th circular polarizer region being arranged opposite the i-thluminescent region for all i=1 to N, each said circular polarizer regionbeing adapted to reflect, within a reflection wavelength rangeΔλ_(cir)(i), the circularly polarized light component the absorbingcircular polarizer is adapted to absorb, and transmit the componentorthogonal thereto;

wherein the reflection wavelength range Δλ_(cir)(i) is adapted tooverlap with the emission wavelength range Δλ_(EL)(i) for all i=1 to N.

The term (wavelength) range, also referred to as band, means any set ofwavelengths, such as one or more distinct continuous intervals on thereal line, and may in general be characterized by a bandwidth and acentral wavelength.

The term ambient light refers, in the context of the invention, to anylight which does not originate from the device itself but from a sourceoutside the device and which adversely affects the contrast of the lightemitted by the electroluminescent device. Which wavelengths have sucheffect depends on the wavelength range at which the electroluminescentdevice is designed to operate. The operating range may in principle beany part of the electromagnetic spectrum, in particular including theinfrared or ultraviolet range. Typically, however, ambient light refersto light to which the human eye is susceptible, that is the visiblerange of the spectrum which extends from about 400 to 700 nm. In thecontext of the present invention it is assumed that the range ofwavelengths emitted by the electroluminescent device,Δλ_(EL)≡∪_(i)Δλ_(EL)(i), and thus each range Δλ_(EL)(i), is a subset(subrange) of the ambient wavelength range.

By arranging a reflective circular polarizer between the luminescentlayer and the absorbing circular polarizer, an electroluminescent devicewhich combines a good contrast, even under high ambient lightingconditions, with a satisfactory brightness and/or efficiency isachieved.

The combination of light-reflecting surface, absorbing circularpolarizer and reflective circular polarizer has, within a range ofoverlap of Δλ_(cir)(i) and Δλ_(EL)(i), the effect that both circularcomponents of the unpolarized light emitted by the luminescent layer areable to exit the electroluminescent device and reach the viewer, whereasof the ambient light incident on the display only one circularcomponent, viz. the component which has the handedness transmitted bythe absorbing and reflective polarizer, is able to reach the viewer.Accordingly, compared to the situation where, in accordance with theprior art, no reflective polarizer but only an absorbing polarizer ispresent, the brightness of the device has improved, whereas the contrastis maintained, where contrast in this respect is defined as thedifference in intensity of emitted light and reflected ambient light.

An electroluminescent device comprising a reflective circular polarizeris known per se from WO 97/12276. The electroluminescent devicedisclosed therein does not comprise an absorbing circular polarizer andis silent in regard the problem of ambient light reflection whichsilence is to be expected as the electroluminescent device disclosed inWO 97/12276 is provided in the form of an illumination system.Furthermore, in WO 97/12276 no reference is made to patterned reflectivepolarizers having distinct reflective circular polarizing regions havingmutually different reflection bands to tune the reflection band to thecorresponding oppositely arranged luminescent region. The reflectivecircular polarizers disclosed in WO 97/12276, both narrow band and broadband, may be used in the electroluminescent device in accordance withthe invention provided its reflection band overlaps with a luminescentregion.

Preferably, in order to further reduce the intensity of reflectedambient light while maintaining brightness, any and, preferably, allreflection bands Δλ_(cir)(i) are tuned to respective emission wavelengthranges Δλ_(EL)(i). Accordingly, in a preferred embodiment of theinvention, the range Δλ_(cir)(i) is adapted to be coincident with or asub-range of the range Δλ_(EL)(i) for any or, preferably, all i=1 to N.Those skilled in the art will appreciate that in practice the emissionspectrum of a luminescent region and the reflection band of a reflectivepolarizer region have intricate shapes and it will in general not bepossible to exactly match the reflection and emission band nor will itbe possible to make the reflection band a subrange of an emission bandin a strict mathematical sense. Tuning of the reflection band to theemission band may be achieved by narrowing the reflection band(s) of thereflective circular polarizer. Preferred are narrow band circularreflective polarizers having regions wherein the reflection wavelengthrange Δλ_(cir)(i) has a bandwidth of 20 to 150 nm, or 40 to 100 nm forany or, preferably, all i=1 to N.

Such narrow band reflective polarizers are conventional and may bemanufactured in a simple reliable manner using methods known in the artper se.

For wavelengths outside the reflection bands Δλ_(cir)(i), the reflectivepolarizer is inoperative while the light-reflecting surface and theabsorbing circular polarizer remain operative. Accordingly, encounteringonly the light-reflecting surface and the absorbing circular polarizer,both circular components of the ambient light incident on the device arecompletely absorbed whereas of the emitted light only one circularcomponent is absorbed. Consequently, if the reflection band Δλ_(cir)(i)of a reflective polarizer region is narrowed by excluding wavelengths atwhich the corresponding luminescent region does not luminesce, thebrightness of that luminescent region is maintained while the intensityof the reflected ambient light is reduced and thus the contrastimproved.

Beyond the point where the range Δλ_(cir)(i) is coincident withΔλ_(EL)(i), that is at the point where Δλ_(cir)(i) becomes a sub-rangeof Δλ_(EL)(i), the brightness of the luminescent region starts todecrease while the contrast and also the color purity continues toimprove. Put differently, if Δλ_(cir)(i) is a sub-range of Δλ_(EL)(i),contrast can be exchanged for brightness depending on the requirementsof the application in which the electroluminescent device is useddemonstrating the versatility of the electroluminescent in accordancewith the invention.

If the reflection band is a narrow band (band width for less than 150nm) or in particular is narrower than the emission band, the reflectionband is preferably positioned on the blue (high energy, smallwavelength) side of the emission band, that is the central wavelength ofreflection band is blue shifted with respect to central wavelength ofemission band. The reflection band is viewing angle dependent, theemission band is not. In particular in going from normal to off-normalviewing angles the reflection band becomes red shifted. With thereflection band positioned on the high energy side of the emission band,the reflection band travels through the emission band in going fromnormal to off-normal viewing angles.

Multi-color and full-color benefit most from the electroluminescent inaccordance with the invention. Therefore, in a preferred embodiment, anelectroluminescent device comprises a first luminescent region adaptedto emit light within an emission wavelength range Δλ_(EL)(L)corresponding to a first color, a second luminescent region adapted toemit light within an emission wavelength range Δλ_(EL)(2) correspondingto a second color, and a patterned reflective circular polarizercomprising a first reflective circular polarizer region having areflection wavelength range Δλ_(cir)(1) for reflecting light of thefirst color and a second reflective circular polarizer region having areflection wavelength range Δλ_(cir)(2) for reflecting light of thesecond color.

Of particular interest are full-color electroluminescent devices whichcomprise, for example, luminescent regions emitting red, green and bluelight respectively and a patterned reflective circular polarizer regioncomprising reflective circular polarizer regions for reflecting red,green and blue light respectively, the luminescent regions beingarranged in general in triplets each consisting of a red, a green and ablue luminescent region to form one full-color picture element. Asdetailed herein below, patterned reflective circular polarizers areknown as such.

The light-reflecting surface causes ambient light reflection which isthe problem the present invention sets out to solve. An obvious solutionwould then be to eliminate the light-reflecting layer from the device.However, in doing so the useful purpose of the light-reflecting layer isalso lost. For example, the light-reflecting layer helps to direct thelight emitted by the luminescent layer, said light emission being ingeneral omni-directional, toward the viewer and thus to increase thebrightness and/or efficiency of the device. In many electroluminescentdevices, the light-reflecting surface is a surface of a part which is afunctional part of the device, for example, the light-reflecting surfacemay be the surface of an electrode of the electroluminescent device. Insuch a case integration of parts is achieved.

In a preferred embodiment, the light-reflecting regions correspond tosurface regions of electrodes of the electroluminescent device.

Generally, electrodes are required in order to enable the luminescentlayer to emit light. This is, for example, the case in organic andpolymeric light emitting diodes. Such diodes require electrodes forinjecting holes and electrons into the luminescent layer. Integration offunctionality is achieved by using the electrode surfaces as thelight-reflecting regions. Moreover, as the electrodes are in generalvery close to the luminescent layer parallax is avoided.

In a particular embodiment, the device comprises a substrate and thereflective circular polarizer is arranged between the said substrate andthe luminescent layer.

In order to facilitate manufacture and/or lend mechanical supportelectroluminescent device generally comprise a substrate. Having toprovide mechanical support, the substrate has a thickness which issubstantially larger then the other layers of the device. If provided onthe side of the light-reflective surface facing from the viewer, thesubstrate may be silicon substrate the surface of which may also serveas the light-reflecting surface. If the substrate is arranged on theviewing side of the light-reflecting surface the substrate is to betransparent. In order to reduce parallax, it is preferable that thecircular reflective polarizer and the luminescent layer are arrangedclose together which is achieved by arranging the substrate on theviewer side of the circular reflective polarizer instead of between theluminescent layer and the reflective polarizer.

In a preferred embodiment of the invention, any or, preferably, all ofthe reflective circular polarizer regions comprises cholesteric materialhaving a helicoidal order. Reflective polarizers are known in the artper se. For example in U.S. Pat. No. 6,025,897 a reflective polarizercomprised of a stack of alternating birefringent and isotropic layers isdisclosed which is adapted to reflect one component of linearlypolarized light and transmit the component orthogonal thereto.Obviously, by arranging the linear reflective linear polarizer betweenconventional quarter wave retarders a reflective circular polarizer isobtained. A further embodiment of a reflective polarizer may be obtainedby deposition of birefringent inorganic materials, such assilicondioxide, on a rotating substrate to form a chiral, twistedcolumnar layer.

As described above, in multi-color or full-color electroluminescentdevices the use of a patterned reflective circular polarizer comprisingreflective circular regions having different reflection bands which areeach tuned to the corresponding luminescent regions is of particularadvantage. Helicoidally ordered cholesteric material is particularlysuitable for obtaining such patterned circular reflective polarizers.Such a circular reflective polarizer and a convenient method ofmanufacturing such a polarizer is for example described in WO 00/34808.

In order to mitigate any viewing angle dependencies introduced by theprovision of the reflective circular polarizer, an electroluminescentdevice in accordance with the invention further comprises, arranged onthe side of the reflective circular polarizer facing away from theluminescent layer, a compensation layer having an optical indicatrixcomplementary to that of the reflective circular polarizer. In case of acholesteric reflective circular polarizer, the compensation layer has aslow axis along its normal and two equal fast axes in the directionsorthogonal to the normal direction.

The invention may be suitably used for any kind of electroluminescentdevice, including in particular, light emitting chemical cells and mono-charge carrier electroluminescent devices (hole-only, electron-only),such as field emission devices. The invention is of particular use inorganic and, more particular, polymeric light emitting diodes as suchdevices in general comprise a metal electrode which has a specularreflectivity.

The electroluminescent device in accordance with the invention may beused for lighting signage applications, but is particularly suitable foruse as a display such as a segmented display or a matrix display of thepassive or active type.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

In the drawings:

FIG. 1, schematically, shows, in a cross-sectional view, an embodimentof an electroluminescent device in accordance with the invention,

FIG. 2 shows, schematically, a trace of a light ray emitted by the ELdevice of FIG. 1,

FIG. 3 shows, schematically, a trace of an ambient light ray incident onthe device of FIG. 1, and

FIG. 4 schematically shows, exemplary wavelength ranges of a luminescentregion and a reflective circular polarizer region.

FIG. 1, schematically, shows, in a cross-sectional view, an embodimentof an electroluminescent device in accordance with the invention. Theelectroluminescent device 1 shown in FIG. 1 comprises a conventionalpolymeric or organic light emitting diode (LED) 13. The use of thepolymeric or organic LED 13 is not essential, any otherelectroluminescent device may be used to illustrate the invention. A LEDis particularly suited to illustrate the invention because it comprisesin general an electrode which specularly reflects (ambient and emitted)light with high efficiency. The light emitting diode 13 comprises,successively:

a substrate 3 which is at least transparent for the light to be emittedby the LED 13;

a first electrode layer 5 for injecting charges of a first type (eitherholes or electrons), the electrode layer 5 being transparent for thelight to be emitted by the LED 13;

a charge-transport layer 7 for transporting charges of the first typefrom the electrode layer 5 to the luminescent layer 9;

a luminescent layer 9 comprising luminescent regions 9 ^(i), i=1 to Nadapted to emit light within an emissive wavelength range Δλ_(EL)(i);

a second electrode layer 11 for injecting charges of a second type(either holes or electrons), the electrode layer 11 being made of metaland having a light-reflecting surface 10 comprising light-reflectingsurface regions 10 ^(i), i=1 to N arranged opposite the luminescentregions 9 ^(i).

Typically, the substrate 3 is adapted to be impervious to oxygen and/ormoisture as in general the active layers of the LED 13 degrade whenexposed to oxygen and/or moisture. The substrate material is as such notcritical, suitable choices being glass, ceramic or plastic or analternating stack of plastic and inorganic layers. Typically, thesubstrate has a thickness of about 0.5 to about 5 mm to providemechanical support. Being arranged on the side of the electrode layer 5which faces away from the luminescent layer 9, the substrate allowsfacilitates easy manufacture. The substrate may alternatively bearranged on the side of the reflective polarizer 17 facing away from theluminescent layer 9. This has the advantage that the reflectivepolarizer 17 is substantially closer to the luminescent layer 9 andaccordingly parallax is reduced. Alternatively, the substrate may bearranged on the side of the light-reflecting surface 10 facing away fromthe luminescent layer 9 allowing non-transparent substrate materials tobe used. A silicon substrate is particularly attractive in this respectas such a substrate allows integration of the electronic circuitryneeded to drive the electroluminescent device 1.

The first electrode layer 5 is adapted to inject charges of a firsttype, which may be holes or electrons, into the luminescent layer 9, inthe present embodiment via the charge-transport layer 7. The electrodelayer 5 is to be transparent for the light emitted by the LED 13 if itis to reach the viewer 2. A suitable material for the electrode layer 5is an indiumtinoxide (ITO) but any other conducting transparent materialmay be used. If combined with conventional luminescent material for usein organic and polymeric light emitting diodes, an ITO electrode layer 5in general is a hole-injecting electrode also referred to as the anode.Alternatively, the electrode layer 5 may be a transparentelectron-injecting electrode layer, materials from which such electrodemay be formed being known as such in the art. The electrode layer 5 maybe a continuous layer suitable in case the LED 13 is to form asheet-like back light (for use in e.g. a liquid crystal display) orother large area lighting and signage device. A continuous layer mayalso be used as a common electrode layer in a segmented light emittingdiode display or an active matrix light emitting diode display.Alternatively, the electrode layer 5 may be applied in accordance with adesired pattern in order to present a viewer 2 with a patterned lightemission to represent an icon or other fixed picture. Alternatively, theelectrode layer 5 may be patterned to form a plurality of independentlyaddressable electrodes which may be suitably used to form a pixelateddisplay such a segmented display or a passive or active matrix display.The electrode layer typically has a thickness of about 50 to 250 nm.

The charge-transport layer 7 is optional and may be formed fromconventional materials, poly-ethylenedioxythiophene (PEDOT),poly-aniline and arylamines such as triphenyldiamine (TPD) being typicalexample in the case of hole-transport layers, oxadiazoles,poly-fluorenes and poly-spiro fluorenes being typical examples ofelectron-transport materials. The LED 13 may contain furthercharge-transport layers. In particular such layers may be arrangedbetween the luminescent layer 9 and the second electrode layer 11. Thecharge-transport layer 7 typically has a thickness of about 50 to 300 nmif applied using a wet deposition method such as spin coating or ink jetprinting or of about 5 to 50 nm if applied using a vapor depositionmethod.

The luminescent layer 9 is conventional and may be formed in aconventional manner. The luminescent layer 9 may be formed from lowmolecular weight compound(s) deposited by means of a vapor depositionmethod, in which case the LED is commonly referred to as organic, orhigh molecular weight material(s) such as polymer(s) deposited by meansof a wet deposition method such as spin coating or ink jet printing. Byselecting a suitable luminescent material from which the luminescentlayer 9 is formed the emission wavelength range Δλ_(EL)(i) (simply alsoreferred to as the color) of a particular luminescent region 9 ^(i) isselected. The luminescent layer 9 may be formed from a continuous layerin which case all luminescent regions 9 ^(i) have the same emissionwavelength range, that is Δλ_(EL)(i)=Δλ_(EL) for all i=1 to N, to obtaina monochrome electroluminescent device or the luminescent layer 9 bepatterned to comprise luminescent regions 9 ^(i) of which the color ismutually different to form a multi-color electroluminescent device. Ifcombined with electrode layers 5 and/or 11 comprising individuallyaddressable electrodes multi-color or full-color pixelated displaysbecome available. In this present embodiment, the luminescent layer 9 ispartitioned into RGB pixels, each RGB pixel comprising three distinctand individually addressable luminescent regions 9′ which emit red (R),green (G) and blue (B) light respectively. The thickness of theluminescent layer is typically about 50 to 500 nm in polymer lightemitting diodes and typically about 10 to 100 nm in organic lightemitting diodes.

The second electrode layer 11 is adapted to inject charge carriers of asecond type, the polarity of the second type being different from thefirst type. Similar to the first electrode layer 5 (as described supra)the second electrode layer may be provided as a continuous layer or apatterned layer which may or may not comprise individually addressableelectrodes.

Because the luminescent layer 9 in general emits light in all directionsand thus also in directions pointing away from the viewer 2, thebrightness of the LED 13 can be improved in a simple manner by includinga light-reflecting surface which redirects light emitted by theluminescent layer 9 towards the viewer 2. Since the electrode layer 11is required anyway for injecting charges into the luminescent layer 9,and an electrode is preferably made of high-conducting material, such alight-reflecting surface is conveniently implemented if the electrodelayer 11 is made of a metal, a metal alloy, a highly doped semiconductorlayer or a combination of such layers. In that case, the electrode layersurface 10 serves as a light-reflecting surface. Because electrodematerial is to be arranged opposite any luminescent region 9 ^(i) ifsuch region is to emit light luminescence each luminescent region 9 ^(i)has a light-reflecting surface 10 ^(i) opposite to it. In case theelectrode layer 11 is to inject electrons a low work function metal suchas calcium, barium, magnesium, indium or aluminum or an alloy comprisingsuch a metal is required. If the electrode layer is to inject holes ahigh work function metal may be suitably used such as gold, silver, oraluminum.

Alternatively, in the latter case, a combination of an ITO layercombined with a light-reflecting surface may also be used.

The LED 13 may be manufactured in a conventional manner, for example byproviding the substrate 3 in succession with layers 5, 7, 9 and 11.

In operation, a voltage source is connected to the electrode layers 5and 11 and, if a suitable voltage is applied, electrons and holes areinjected into the luminescent layer 9 in which light emission takesplace by means of recombination of holes and electrons.

It is readily apparent form FIG. 1 that, if the light emitting diode 13is used as such, that is without the layers 15 through 23, thelight-reflecting surface 10 not only reflects light emitted by theluminescent layer 9 but also reflects ambient light with highefficiency. This reduces the contrast of the electroluminescent device 1when used under high intensity ambient lighting conditions.

In order to obtain an electroluminescent device which combines a goodcontrast and brightness even when ambient light of high intensity isincident on the electroluminescent device, the electroluminescent device1 comprises in succession:

an absorbing circular polarizer 23 which, being formed from a laminateof a quarter lambda wave layer 19 and a dichroic linear polarizer 21, isadapted to absorb one circularly polarized light component and transmitthe light component orthogonal thereto; and

a reflective circular polarizer 17 comprising reflective circularpolarizer regions 17 ^(i) for i=1, . . . , N, with N>=1, the circularpolarizer region 17 ^(i) being arranged opposite the luminescent region9 ^(i) for all i=I to N, each said circular polarizer region 17 ^(i)being adapted to reflect, within a wavelength range Δλ_(cir)(i), thecircularly polarized light component the absorbing circular polarizer isadapted to absorb, and transmit the component orthogonal thereto,

wherein the wavelength range Δλ_(cir)(i) is adapted to overlap with therange Δλ_(EL)(i) for all i=1 to N.

The electroluminescent device 1 further comprises adhesive layers 15 tojoin the LED 13 and the layers 17, 19 and 21 together to obtain anintegrated structure. The adhesive may be made of optical glue, opticalglues being well known in the art. Although instrumental in reducingparasitic reflections at interfaces between the various layers, theadhesive layer is not essential for the invention. Alternatively, byselecting proper materials, both thermosetting and thermoplasticpolymeric material are useful in this respect, and methods of processingsuch materials, the layers 3, 17, 19, and/or 21 may be firmly joinedwithout adhesive layer.

The quarter wave retarder 19 and the dichroic linear polarizer 21 areboth components well known in the art. It is also well established thatthe combination of a quarter wave retarder and a dichroic linearpolarizer provides an absorbing circular polarizer. Typically, thelinear polarizer 21 is adapted such that the absorbing circularpolarizer is active across the entire wavelength range of the ambientlight incident on the electroluminescent device 1.

Reflective circular polarizers are known as such in the art and any oneof such known reflective circular polarizers may be suitably used in theEL device in accordance with the invention. Examples of suitablereflective polarizers are summarized in WO 97/12276. A furtherembodiment of a reflective polarizer may be obtained by deposition of abirefringent inorganic material, such as silicondioxide, on a rotatingsubstrate to form a chiral, twisted columnar layer such as described inQ. Wu, I. J. Hodgkinson & A. Lakhtakia, ‘Circular polarization filtersmade of chiral sculptured thin films: experimental and simulationresults,’ Optical Engineering, 39, 2000, 1863–1868.

The reflective circular polarizer 17 is patterned to comprise reflectivecircular polarizer regions 17 ^(i) where the reflective circularpolarizer region 17 ^(i) is arranged opposite the luminescent region 9^(i). The reflection band Δλ_(cir)(i) of each of the reflective circularpolarizer regions 17 ^(i) is tuned to the emission band of thecorresponding luminescent region 9 ^(i). Specifically, in the presentembodiment, the reflective circular polarizer regions which are oppositethe blue-emitting luminescent regions have a reflection band forreflecting blue, those opposite a green-emitting luminescent region havea reflection band for reflecting green light, and those opposite ared-emitting luminescent region have a reflection band for reflectingred light.

Patterned reflective circular polarizers such as the patternedreflective circular polarizer 17 are known as such. For example, WO00/34808 discloses a patterned reflective circular polarizer formed fromhelicoidally ordered cholesteric material. Generally, a circularpolarizer formed from helicoidally ordered cholesteric material is alsoknown in the art as a cholesteric polarizer or a chiral nematicpolarizer. Patterned reflective circular polarizers are also known ascholesteric color filters.

In general, cholesteric (chiral nematic) polarizers are formed fromcholesteric (chiral nematic) material which is helicoidally ordered.That is, on a molecular level, the material has a helical symmetrycharacterized by a helix having a helical axis, a handedness and apitch. As is well known helicoidally ordered material reflectscircularly polarized light having the handedness of the helix andtransmits light having the opposite handedness in a wavelength rangehaving a central wavelength Δ_(cir)=n.p and a bandwidth Δλ_(cir)=Δn.p,where p is the pitch of the helix, n the average refractive index of thecholesteric material and Δn the birefringence associated with thecholesteric material. Generally, to be useful as a cholesteric polarizerthe cholesteric material is provided in the form of a layer and isordered on a macroscopic level such that within regions 17 ^(i) thehelical axis extends transversely to the layer.

A preferred chiral nematic (cholesteric) reflective circular polarizeris one obtainable by orienting polymerizable or, more specific,photo-polymerizable, liquid crystalline chiral nematic material toobtain a helicoidally ordered (photo)-polymerizable liquid crystallinechiral nematic layer which is then (photo)-polymerized, whilemaintaining the helicoidal order, to obtain a (photo)-polymerized chiralnematic reflective circular polarizer.

Liquid crystalline chiral nematic compositions which are(photo)-polymerizable are well known in the art (see e.g. WO 00/34808,EP 606940 and EP 982605) and generally comprise a nematic materialwhich, if oriented neat, is capable of forming a uniaxially orientedstate characterized by a birefringence Δn and a chiral compound, whichneed not be itself liquid crystalline, but if added to the nematicmaterial modifies the nematic material into a material which is capableof forming a cholesteric order. The pitch p of a chiral nematiccomposition can be adjusted by the type of chiral compound used and theamount in which it is added. The composition, which preferably furthercontains a photo-initiator to facilitate polymerization, is rendered(photo)-polymerizable by providing the nematic and/or chiral materialwith (photo)-polymerizable groups such as acrylates. Of particularadvantage are crosslinkable chiral nematic compositions. By means of thephoto-polymerizable chiral nematic compositions, reflective circularpolarizers having a variety of band widths become available in a simplemanner and accordingly the band width can be tuned to the emissionwavelength range of the luminescent layer 9 which as is explained belowof particular advantage to improve the contrast of theelectroluminescent device.

Photo-polymerizable chiral nematic compositions are particularlysuitable to manufacture patterned cholesteric reflective polarizers suchas the polarizer 17.

Several methods exist to manufacture patterned cholesteric reflectivepolarizers from photo-polymerizable compositions. In a first method, afirst cholesteric layer is pattern-wise photo-polymerized, e.g. by meansof a mask, to obtain a patterned cholesteric reflective layer havingregions with a first reflection band Δλ^(cir)(1), a second cholestericlayer is pattern-wise photo-polymerized, e.g. by means of a mask, toobtain a second patterned cholesteric reflective layer having regionswith a second reflection band Δλ^(cir)(2), the first and second bandbeing different, and then laminating the first and second cholestericlayer together. The masks are designed such that the first and secondregions do not overlap or at least do not completely coincide. In asecond method, a single layer of photo-polymerizable chiral nematiccomposition is pattern-wise photo-polymerized a number of times, eachtime with a different pattern and each time at a different temperatureexploiting the well known fact that the pitch associated with thehelicoidal order depends on temperature. In a third method use is madeof a photo-polymerizable chiral nematic composition in which the chiralcompound has a photo-adjustable helical twisting power, details of whichare provided in e.g. WO 00/34808.

An even further improvement on contrast and/or color purity is obtainedin an electroluminescent device comprising a reflective circularpolarizer having a reflective circular polarizer region which comprisesone or more dyes which selectively absorb ambient light outside thereflection wavelength range of the said reflective circular polarizerregion. Such reflective circular polarizers are known per se, see WO00/33129.

When the electroluminescent device 1 is connected, via the electrodelayers 5 and 9, to a voltage source and a suitable voltage is applied,holes and electrons are injected into the luminescent layer 9 and byrecombination of holes and electrons light photons are produced whichexit the device 1 via the substrate 3.

More specifically, referring to FIG. 2, an unpolarized light ray 2Aemitted in the direction of the viewer 2 by a luminescent region 9 ^(i)is incident on the reflective polarizer region 17 ^(i). The wavelengthof emitted light ray 2A being within the reflection band of thereflective polarizer region 17 ^(i), the unpolarized light ray 2A issplit into a transmitted right-handed circularly polarized (RH) lightray 2B and a reflected left-handed circularly polarized (LH) light ray2C.

In accordance with the invention, the reflective polarizer 17 and theabsorbing circular polarizer 23 are transmissive for circularlypolarized light of the same handedness. The light ray 2B is thereforetransmitted by the absorbing circular polarizer 23 and able to reach theviewer 2, which after having passed the absorbing circular polarizer 23is linearly polarized.

The left-handed circularly polarized reflected light ray 2C istransmitted by the luminescent region 9 ^(i) and is then incident on thelight-reflecting surface 10. The light-reflecting surface is the surfaceof a metal electrode layer 5. Because the luminescent layer 9 is verythin, about 100 nm, and is to be very uniform in thickness in order toachieve uniform light emission, the metal surface is very smooth. Infact it is smooth to the extent that it specularly reflects light withhigh efficiency. It is well known that a specularly reflecting metalsurface upon reflection inverts the handedness of circularly polarizedlight incident thereon. Consequently, upon reflection of the light ray2C, the reflected light ray is right-handed circularly polarized. Asexplained above with reference to light ray 2B, right-handed circularlypolarized light is transmitted by the reflective and absorbing polarizerto reach the viewer 2. Analogously, light emitted by the luminescentlayer 9 which is directed initially towards the light-reflecting surface10 also reaches the viewer 2. In summary, in the electroluminescentdevice 1 100% (any light loss caused by non-idealities associated withthe components of the device being disregarded) light emitted by theluminescent layer is able to reach the viewer 2.

FIG. 3 shows, schematically, a trace of an ambient light ray incident onthe device of FIG. 1. The unpolarized ambient light ray 3A is filteredby the absorbing circular polarizer 23, the left-handed polarizationbeing absorbed and the right-handed polarization being transmitted toproduce right-handed (RH) polarized light. As shown by the trace of thelight rays 3B, 3C and 3D, the transmitted right-handed light ray 3A isultimately able to reach the viewer 2. The net effect on ambient lightis that 59% of the incident light is absorbed. As the emitted lightefficiency is 100% the contrast of the device is improved compared tothe situation where the reflective and absorbing circular polarizers areabsent.

The intensity of reflected ambient light may be further reduced whilemaintaining the same level of brightness, and thus the contrast, bytuning any and preferably all ranges Δλ^(cir)(i) to respectivewavelength ranges Δλ^(EL)(i).

This is illustrated using FIG. 4 and Table 1.

FIG. 4 schematically shows, exemplary wavelength ranges of a luminescentregion and a reflective circular polarizer region. A typical activewavelength range of an absorbing circular polarizer is also included.

Table 1 lists the relative amounts of ambient light and emitted lightwhich is able to reach the viewer 2 for the wavelength ranges indicatedin FIG. 4.

TABLE 1 ambient light emitted light right-handed left-handedright-handed left-handed I 50% 0% 50% 50% II 0% 0% 50%  0% III 50% 0% —— IV 0% 0% — — V 50% 50% 50% 50%Range V, which corresponds to the situation where both the absorbingpolarizer 23 and the reflective circular polarizer 17 are absent, istaken as a reference.

From Table 1 and FIG. 4 it is apparent that the intensity of ambientlight can be reduced without affecting the intensity of the emittedlight, and thus the contrast improved, by increasing the range IV at theexpense of range III, the optimum being that range III vanishes.Referring to FIG. 4, increasing the range IV at the expense of range IIIamounts to tuning the bandwidth of the reflective polarizer region tothat of the luminescent region by making the reflection bandsubstantially coincident with or substantially a sub-range of theemission band. More specifically, in order to improve on the contrastwhile maintaining the emitted light intensity of a luminescent region 9^(i) at the same level, the corresponding range Δλ^(cir)(i) is to beadapted to be, at least substantially, coincident with, or be,substantially, a sub-range of the range Δλ_(EL)(i). Evidently, the moreluminescent regions and reflective polarizer regions are so tuned, themore substantial the improvement is, the improvement being optimal ifall are so tuned. A manner to achieve the tuning of the reflection bandto the emission wavelength band is to narrow the reflection band of thecircular reflective polarizer regions to a bandwidth of 20 to 150 nm, or40 to 100 nm. As explained above, with respect to cholesteric polarizersband narrowing may be achieved using proper selection of the cholestericmaterial form which the polarizer is formed.

If the reflection band is a narrow band (band width for less than 150nm) or in particular is narrower than the emission band, the reflectionband is preferably positioned on the blue (high energy, smallwavelength) side of the emission band. The reflection band is viewingangle dependent, the emission band is not. In particular in going fromnormal to off-normal viewing angles the reflection band becomes redshifted. With the reflection band positioned on the high energy side ofthe emission band, the reflection band travels through the emission bandin going from normal to off-normal viewing angles.

From Table 1 and FIG. 4 it is furthermore apparent that the balancebetween the emitted light intensity and contrast may be adjusted byexchanging wavelengths between ranges I and II. In particular,increasing range I at the expense of II increases the emitted lightintensity but reduces contrast and vice versa. Put differently, if thereflection band of the reflective polarizer region is made narrower(within the emission wavelength band), contrast improves at the expenseof brightness. Making the reflection band narrower also improves thecolor purity of the luminescent region because, within the reflectionband, the emission spectrum relatively sharpens due to the reflectivepolarizer region, 100% of the emitted light reaching the viewer withinand only 50% outside the reflection band.

From Table 1 and FIG. 4 it is also apparent that narrowing the emissivewavelength band of a luminescent region, which may achieved e.g. byusing a different luminescent material, improves the contrast. Thisimprovement in contrast is not necessarily obtained at the expense ofemitted light intensity. Therefore, the invention is particularlysuitable for electroluminescent device which use narrow emissivewavelength bands. Color monochrome multi-color or full-color typicallypreferably use such narrow emissive wavelength bands. In particular, inelectroluminescent displays having independently addressable regions theuse of narrow emissive ranges is desirable as these provide a high colorpurity and saturation.

The absorbing circular polarizer and in particular the reflectivecircular polarizer 17 is made of optically anisotropic material. Theoptical anisotropy may be characterized by an optical indicatrix, athree by three matrix whose diagonal elements, when presented indiagonal form, represent the refractive index in three principalmutually orthogonal directions. The optical anisotropy causes therefractive indices experienced by normally and obliquely incident lightrays to be different. One result of this is that, for example, thereflection band of the reflective circular polarizer becomes red-shiftedunder oblique incidence. In display applications such angular dependentoptical behavior is known as viewing angle dependency and is consideredundesirable. Also, under oblique incidence the contrast may be less.

Viewing angle dependency is mitigated if the electroluminescent device 1comprises a reflective circular polarizer having a reflective circularpolarizer region which comprises one or more dyes which selectivelyabsorb ambient light outside the reflection wavelength range of the saidreflective circular polarizer region. Such reflective circularpolarizers are known per se, see WO 00/33129.

Viewing angle dependency is further mitigated if the electroluminescentdevice 1 comprises, arranged on the side of the reflective circularpolarizer facing away from the luminescent layer, a compensation layer(not shown) having an optical indicatrix complementary to that of thereflective circular polarizer, in particular in case of a cholestericreflective circular polarizer, the compensation layer has a fast axisalong its normal and two equal slow axes in the directions orthogonal tothe normal direction.

Such compensation layers and methods of manufacturing such layers arewell known in the art. The known compensation layers can be suitablyused in the context of the present invention provided the opticalindicatrix is adapted to compensate for the particular reflectivecircular polarizer of the electroluminescent device 1. For example, areflective circular polarizer or, more particular a reflectivecholesteric polarizer, may be conveniently compensated using acompensation layer comprising homeotropically aligned nematic materialsuch as a polymer or a polymer network.

1. An electroluminescent device comprising: an electroluminescent layerhaving luminescent regions i=1 to N, with N>=1, each adapted to emitlight within an emission wavelength range Δλ_(EL)(i); a light-reflectingsurface comprising one or more light-reflecting surface regionsarranged, with a light-reflecting side, opposite respective ones of theluminescent regions, the surface regions being capable of inverting thehandedness of a circularly polarized component of light incident on thesurface regions; arranged on a side of the electroluminescent layerfacing away from the light-reflecting surface, an absorbing circularpolarizer adapted to absorb one circularly polarized light component andtransmit the light component orthogonal thereto; and arranged betweenthe absorbing circular polarizer and the electroluminescent layer, areflective circular polarizer comprising reflective circular polarizerregions i=1, . . . , N, with N>=1, the i-th circular polarizer regionbeing arranged opposite the i-th luminescent region for all i=1 to N,each circular polarizer region being adapted to reflect, within areflection wavelength range Δλ_(cir)(i), the circularly polarized lightcomponent the absorbing circular polarizer is adapted to absorb, andtransmit the component orthogonal thereto; wherein the reflectionwavelength range Δλ_(cir)(i) is a sub-range of the emission wavelengthrange Δλ_(EL)(i) for at least one region of the regions i=1 to N.
 2. Theelectroluminescent device of claim 1, wherein the reflection wavelengthrange Δλ_(cir)(i) of the at least one region has a bandwidth of 20 to150 nm.
 3. The electroluminescent device of claim 1, including a firstluminescent region adapted to emit light within an emission wavelengthrange Δλ_(EL)(1) corresponding to a first color, a second luminescentregion adapted to emit light within an emission wavelength rangeΔλ_(EL)(2) corresponding to a second color, and a patterned reflectivecircular polarizer comprising a first reflective circular polarizerregion having a reflection wavelength range Δλ_(cir)(1) for reflectinglight of the first color and a second reflective circular polarizerregion having a reflection wavelength range Δλ_(cir)(2) for reflectinglight of the second color.
 4. The electroluminescent device of claim 1,wherein the light-reflecting regions correspond to surface regions ofelectrodes of the electroluminescent device.
 5. The electroluminescentdevice of claim 1, including a substrate, and wherein the reflectivecircular polarizer is arranged between the substrate and the luminescentlayer.
 6. The electroluminescent device of claim 1, wherein at least onereflective circular polarizer region comprises cholesteric materialhaving a helicoidal order.
 7. The electroluminescent device of claim 1,including, arranged on the side of the reflective circular polarizerfacing away from the luminescent layer, a compensation layer having anoptical indicatrix complementary to that of the reflective circularpolarizer.
 8. The electroluminescent device of claim 1, wherein theelectroluminescent device includes an organic or a polymer lightemitting diode.
 9. The electroluminescent device of claim 1, wherein thereflection wavelength range Δλ_(cir)(i) of the at least one region has abandwidth of 40 to 100 nm.
 10. The electroluminescent device of claim 1,wherein the reflection wavelength range of the at least one region issubstantially narrower than the corresponding emission wavelength range.11. The electroluminescent device of claim 10, wherein the reflectionwavelength range of the at least one region is positioned at a lowerwavelength side of the emission wavelength range.
 12. Theelectroluminescent device of claim 1, wherein the reflection wavelengthrange Δλ_(cir)(i) is a sub-range of the emission wavelength rangeΔλ_(EL)(i) for all of the regions i=1 to N.
 13. The electroluminescentdevice of claim 12, wherein the reflection wavelength range of each ofthe regions is substantially narrower than the corresponding emissionwavelength range.
 14. The electroluminescent device of claim 13, whereinthe reflection wavelength range of each of the regions is positioned ata lower wavelength side of the corresponding emission wavelength range.15. The electroluminescent device of claim 12, wherein the reflectionwavelength range Δλ_(cir)(i) of each region has a bandwidth of 20 to 150nm.
 16. The electroluminescent device of claim 12, including a firstluminescent region adapted to emit light within an emission wavelengthrange Δλ_(EL)(1) corresponding to a first color, a second luminescentregion adapted to emit light within an emission wavelength rangeΔλ_(EL)(2) corresponding to a second color, and a patterned reflectivecircular polarizer comprising a first reflective circular polarizerregion having a reflection wavelength range Δλ_(cir)(1) for reflectinglight of the first color and a second reflective circular polarizerregion having a reflection wavelength range Δλ_(cir)(2) for reflectinglight of the second color.
 17. The electroluminescent device of claim12, wherein the light-reflecting regions correspond to surface regionsof electrodes of the electroluminescent device.
 18. Theelectroluminescent device of claim 12, including a substrate, andwherein the reflective circular polarizer is arranged between thesubstrate and the luminescent layer.
 19. The electroluminescent deviceof claim 12, wherein each reflective circular polarizer region comprisescholesteric material having a helicoidal order.
 20. Theelectroluminescent device of claim 12, wherein the electroluminescentdevice includes an organic or a polymer light emitting diode.